U.S. Pat. No. 4,242,183 discloses a type of silver cathode uniquely useful for the electrolytic reduction of tri- and tetrachloropicolinic acids in basic aqueous solution to the 3,6-dichloroacid. (The latter process is disclosed in detail in U.S. Pat. No. 4,217,185.) According to the '183 patent, the cathode is prepared for use by cleaning it with 1:1 water and concentrated HCl, anodizing it in aqueous base to form a surface layer of silver oxides and then electrolytically reducing the oxide layer in a specific manner to convert it to a "spongy" layer of silver microcrystals wet with aqueous base.
The activity of the cathode is impaired by formation thereon of deposits derived from cations of base metals (such as iron, nickel, copper, etc.) which may be present in the reagents used and/or may be products of corrosion of the anode or cell walls. Such deposits can be at least largely removed and the cathode activity correspondingly restored by rewashing the cathode with aqueous HCl, re-anodizing, etc. However, this procedure results in co-dissolution of silver in amounts which, after repeated reactivations of the cathode, represent a not-insubstantial expense.
Thus, a more selective agent than hydrochloric acid for removing base metal-derived deposits from silver cathode surfaces is highly desirable. Of the various reagents which might be considered, other mineral acids which are capable of dissolving base metals and/or their oxides at practicable rates are the most logical candidates as economic reagents.
It can be expected that silver in contact with the periphery of a surface deposit of (or derived from) a base metal will exhibit a different corrosion behavior with respect to a given acid than will silver in contact only with silver. Nevertheless, that which is known about the resistance of silver per se to corrosion by aq. HCl and other base metal-attacking acids could be expected to be helpful in choosing a possibly more selective acid. However, the meager information found in the literature suggests that hydrochloric acid is less corrosive to silver than the other common mineral acids.
U. R. Evans implies (The Corrosion and Oxidation of Metals: Scientific Principles and Practical Applications; Ed. Arnold Ltd., London, page 348 (1960)) that silver is generally more resistant to hydrochloric acid than to sulphuric acid by stating that "The use of silver to resist hydrochloric acid and of lead to resist sulphuric acid are connected with the low solubilities of silver chloride and lead sulphate." Similarly, according to The Corrosion Handbook (Uhlig, ed.), John Wiley & Sons, Inc., N.Y., pages 316-319 (1948) silver immersed for 24 hours in 30% HCl and 20% H.sub.2 SO.sub.4 at their boiling points, under air, loses weight to the extent of 0.35 and 4.0 MDD (milligrams per square decimeter per day), respectively, and immersion in 50% H.sub.3 PO.sub.4 at its boiling point, under air, results in a weight gain in the amount of 21.0 MDD (in the form of an adherent coating). Data is also given for 24 hour immersion in concentrated HCl (1.00 MDD), 10% H.sub.2 SO.sub.4 (2.4 MDD) and 60% H.sub.2 SO.sub.4 (26.0 MDD) at their boiling points. (HF and HNO.sub.3 are well known to be highly corrosive to all but the noblest metals; no data for corrosion of silver by HBr was found.)
Since corrosion mechanisms are complex and dependent on a number of variables, teachings of the foregoing type do not foreclose the possibility that sulfuric or phosphoric acid might prove to be more selective than hydrochloric acid in application to base metal deposits on silver cathodes. However, they are certainly not encouraging in that regard.